This invention relates to a diode laser system and, more particularly, a high power diode laser system.
Laser diodes are well known as reasonably priced, small and robust sources of laser beams. Conventional laser diodes with small output power and good coherence properties have been available, and they are used in many applications such as CD players, bar-code readers etc.
More recently, laser diodes with several Watts of output power have become available. These high-power laser diodes are potentially applicable in industrial areas requiring high power light sources, such as in printing, material processing, medicine, optical sensors and pumping of high-power single mode active waveguides. However, as the optical quality of these lasers is not sufficiently good for many of these applications, it has been a long felt need to improve the optical quality of high-power laser diodes. FIGS. 1a-b schematically illustrate such a broad area laser comprising a semiconductor diode 100 with a p-n junction 101 which provides an amplifying medium when a pump current is applied to the junction via electrodes 108 and 109. In a conventional laser diode, two opposing end facets 102 and 103 are coated with a reflecting coating, thereby generating a laser cavity between them. If one of the end facets 103 is totally reflecting and the other end facet 102 has a low reflectivity, typically in the range 4-8%, a laser beam with an angular distribution around the optical axis 104 is emitted. GaAlAs semiconductor broad-area laser diode arrays are attractive, because they can be operated with a low voltage and because they have a lifetime of more than 20.000 hours. The optical output power of these diodes is related to the dimensions of the light emitting output facet, in particular of the area 105 of the light emitting region which will be referred to as transverse gain area. Commercially available broad area lasers have light-emitting areas of a few micrometers in the direction 106 across the active region, e.g. 1-2 μm. In the lateral direction 107 along the active region, the so-called stripe width may be varied. By increasing the stripe width of the output facet the output power has increased significantly during the last 10 years. Broad-area lasers with 200 μm stripe width are now commercially available with an output power of up to 4 W. However, it is a problem of these lasers that the dimensions of the output facet may not be increased arbitrarily, since an increase of the stripe width causes undesired side effects which decrease the quality of the laser diode. These side effects include filamentation/self-focussing, thermal rollover, low power- and frequency stability, and poor spatial and temporal coherence. Filamentation or self-focussing inside the active semiconductor material causes very poor spatial beam quality of conventional broad area diodes. Filamentation is caused by high intensity spatial regions, so-called hot spots, which are formed inside the laser cavity due to the nonlinear optical interaction between the semiconductor material and the laser beam. These localized hot spots are formed in unpredictable patterns causing unpredictable phase modulations of the laser light, thereby severely reducing the quality of the laser field profile. The term thermal rollover refers to the problem that the optical output power from broad area lasers is limited by the so-called catastrophic facet damage caused by thermal runaway and melting of the cleaved laser mirror. Power and frequency instabilities occur if the intensity is above a critical intensity.
Attempts have been made to improve the quality of the output beams of broad-area diodes, e.g. by providing external feedback from an external cavity (see e.g. C. Chang-Hasnain, D. F. Welch, D. R. Scifres, J. R. Whinnery, A Dienes, and R. D. Burnham, “Diffraction-limited emission from a diode laser array in an apertured graded-index external cavity,” Appl. Phys. Lett. 49, 614-616 (1986)). However, in general, diode lasers with feedback are very sensitive to vibrations in the external mirror and therefore the external cavity must be stabilized. Furthermore, bifurcations and chaotic behaviour may take place in the output beam of feedback diode lasers if the amount of feedback is above a critical value.
The light emitted by conventional broad area lasers comprises a number of spatial modes, where each mode corresponds to a respective angle of emission. The lowest order mode, the so-called fundamental mode, is emitted in the direction of the optical axis 104, while the higher-order modes are emitted as a twin-lobe intensity distribution along the so-called low-coherence axis, i.e. the x-axis in FIGS. 1a-b. The lobes of any given higher-order mode are emitted at corresponding angles on respective sides of the optical axis. The above mode structure influences the coherence properties of the emitted laser beam. In particular, when operated high above threshold, broad area lasers oscillate in a high number of spatial modes with each mode radiated in different directions. Consequently, the spatial coherence is rather poor. High power broad area lasers oscillate in many longitudinal modes and the temporal coherence of the laser is very low, typically some hundred microns. The low-coherence axis is also referred to as slow axis, while the axis perpendicular to the slow axis, i.e. the y-axis in FIGS. 1a-b is referred to as the fast axis.
If the stripe width of a broad area laser is increased beyond 200 μm, the degradation of the laser light due to the above side effects overcompensate for the increase in output power and the properties of the output beam become very poor. In particular, laser diodes with large light light-emitting areas have poor spatial coherence properties in the lateral direction of the light-emitting aperture, the so-called low-coherency axis. Due to this disadvantage, the resulting light beam cannot be focused to a small spot size over long distances. A measure of quality used to estimate the spatial coherence of laser sources is the so-called M2 value. The M2 value is related to a light source's ability to be focused.
Robert J. Lang, K. Dzurko, Amos A. Hardy, Scott Demard, Alexander Schoenfelder, and David F. Welch, “Theory of Grating-Confined Broad-Area Lasers” (IEEE Journal of Quantum Electronics, Vol. 34, No. 11, November 1998, p. 2196-2210) describe a so-called angled-grating distributed feedback laser (α-DFB) which comprises a broad-area gain-stripe with a permanently embedded grating parallel to the gain stripe such that the stripe and the grating are disoriented from the cleaved facets by a substantial angle. The described α-DFB supplies both feedback and selective spatial filtering, enforcing single-spatial mode oscillation.
However, it is a disadvantage of the above prior art system that it is difficult and cost-intensive to fabricate laser diodes with embedded gratings. It is a further disadvantage of this system that thermal effects influence the alignment of the laser and therefore it does not lead to stable long term operation.